Forum: GNU Radio Frequency dependent problem after syncronizing ,Please help!

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8609964d75de56552443da57880d4424?d=identicon&s=25 Bruh G. (bruh)
on 2009-02-12 14:43
Hey all,

I syncronized my USRPs(one is v4.3 and the other v4.5) by using the USRP
clocking notes on http://www.gnuradio.org/trac/wiki/USRPClockingNotes,
to make them frequency and phase syncronized.

I want to use one as TX and other as RX, with XCVR daugherboards on
them.
But, what I got afre syncronizing is a wierd signal with very small peak
at the center frequency;moreover its whole spectrum level and shape
changes randomly with time . I checked that these problem is not a
reception problem; rather it is a problem created when the master and
slave are set at the same frequency. I check this by making the
following experiment.

(I transmit a carrier @4.96GHz from the slave USRP and a carrier @5GHz
from a signal generator.I verify that both transmissions are working
using a spectrum analyser.Then,I run usrp_fft.py on the master USRP.I
see that the master USRP receives the 5GHz transmission from signal
generator without problem, but when i change the center frequency to
4.96GHz to receive from the slave, I got a weird signal.The signal is
not stable, the noise(whole spectrum) level decreases and increases very
fast and it has only a very small peak at the center frequency.Moreover,
the spectrum changes very much with gain.)

I also tried the reverse(using master as transmitter and slave as
receiver) and also changing the carrier frequencies.But, these weird
behavior remains.

So,anyone having similar problems before? please could you help me with
this issue?

Thanks for your help!

Bruhtesfa
D0072e69d706bb3ca211d33a1b536e2c?d=identicon&s=25 Johnathan Corgan (Guest)
on 2009-02-12 19:34
(Received via mailing list)
On Thu, Feb 12, 2009 at 5:43 AM, Bruhtesfa Ebrahim
<lists@ruby-forum.com> wrote:

> I syncronized my USRPs(one is v4.3 and the other v4.5) by using the USRP
> clocking notes on http://www.gnuradio.org/trac/wiki/USRPClockingNotes,
> to make them frequency and phase syncronized.

Ok.

> ...Then,I run usrp_fft.py on the master USRP.I
> see that the master USRP receives the 5GHz transmission from signal
> generator without problem, but when i change the center frequency to
> 4.96GHz to receive from the slave, I got a weird signal.The signal is
> not stable, the noise(whole spectrum) level decreases and increases very
> fast and it has only a very small peak at the center frequency.Moreover,
> the spectrum changes very much with gain.)

It sounds like, since you have perfect synchronization between
transmitter and receiver, that what you have after downconversion in
the daughterboard is DC + noise.  That is, the transmitted carrier
(plus noise and multipath from the channel) is being mixed with a
synchronized local oscillator, resulting in only the noise components
being left in the complex baseband signal GNU Radio is receiving.
There will also be a DC component based on the path delay and
wavelength causing a phase shift between the transmitted carrier phase
and and the receiver LO phase.

Johnathan
8609964d75de56552443da57880d4424?d=identicon&s=25 Bruh G. (bruh)
on 2009-02-12 22:20
Johnathan Corgan wrote:
> On Thu, Feb 12, 2009 at 5:43 AM, Bruhtesfa Ebrahim
> <lists@ruby-forum.com> wrote:

> It sounds like, since you have perfect synchronization between
> transmitter and receiver, that what you have after downconversion in
> the daughterboard is DC + noise.  That is, the transmitted carrier
> (plus noise and multipath from the channel) is being mixed with a
> synchronized local oscillator, resulting in only the noise components
> being left in the complex baseband signal GNU Radio is receiving.
> There will also be a DC component based on the path delay and
> wavelength causing a phase shift between the transmitted carrier phase
> and and the receiver LO phase.
>
> Johnathan



Thank you Johnathan!

What makes it strange is the dependence on gain.when the linear gain in
the spectrum panel(usrp_fft.py)is set to 0,the signal at the center(the
DC)is 5dB higher than the noise level.When I increase the gain, the
difference between the DC and noise level keeps decreasing and finally
the DC level is equal to the noise level.
Specifically, for a linear gain >=20, the signal around DC is not
visible(it is equal to the noise level).

is this gain dependence normal?

Bruhtesfa
D0072e69d706bb3ca211d33a1b536e2c?d=identicon&s=25 Johnathan Corgan (Guest)
on 2009-02-12 23:18
(Received via mailing list)
On Thu, Feb 12, 2009 at 1:20 PM, Bruhtesfa Ebrahim
<lists@ruby-forum.com> wrote:

> What makes it strange is the dependence on gain.when the linear gain in
> the spectrum panel(usrp_fft.py)is set to 0,the signal at the center(the
> DC)is 5dB higher than the noise level.When I increase the gain, the
> difference between the DC and noise level keeps decreasing and finally
> the DC level is equal to the noise level.
> Specifically, for a linear gain >=20, the signal around DC is not
> visible(it is equal to the noise level).

Actually, what you're now describing is more like not receiving any
signal at all--the small line at DC is residual DC offset from the
ADC, and the noise is simply that, noise that gets amplified according
to the gain setting.  At a certain point the noise power exceeds the
small DC offset.

Johnathan
7de99ba7a8144e8f34524dc0ca683bd7?d=identicon&s=25 unknown (Guest)
on 2009-02-12 23:24
(Received via mailing list)
Gnuradio was recently mentioned in this article on the technology of
phone
snooping:

http://news.cnet.com/8301-13739_3-10159055-46.html
745d8202ef5a58c1058d0e5395a78f9c?d=identicon&s=25 Eric Blossom (Guest)
on 2009-02-16 21:55
(Received via mailing list)
On Thu, Feb 12, 2009 at 02:17:05PM -0800, Johnathan Corgan wrote:
> Actually, what you're now describing is more like not receiving any
> signal at all--the small line at DC is residual DC offset from the
> ADC, and the noise is simply that, noise that gets amplified according
> to the gain setting.  At a certain point the noise power exceeds the
> small DC offset.
>
> Johnathan

Agreed about no (or low) signal, though I think the small line at DC
is a truncation artifact in the FPGA.

Eric
8609964d75de56552443da57880d4424?d=identicon&s=25 Bruh G. (bruh)
on 2009-02-16 22:10
Eric Blossom wrote:
> On Thu, Feb 12, 2009 at 02:17:05PM -0800, Johnathan Corgan wrote:
>> Actually, what you're now describing is more like not receiving any
>> signal at all--the small line at DC is residual DC offset from the
>> ADC, and the noise is simply that, noise that gets amplified according
>> to the gain setting.  At a certain point the noise power exceeds the
>> small DC offset.
>>
>> Johnathan
>
> Agreed about no (or low) signal, though I think the small line at DC
> is a truncation artifact in the FPGA.
>
> Eric

Hi Eric,

Is there any DC(low frequency)removing/coupling effect in the USRP that
arises due to syncronization?
 Because, before I syncronize the boards i was able to transmit,for
instance at 4.96GHz and receive a narrow band carrier at the same
frequency,4.96GHz. Now, I am able to see the correct spectrum if and
only if i receive at a frequency offset greater than +/- 1KHz.

Bruhtesfa
8609964d75de56552443da57880d4424?d=identicon&s=25 Bruh G. (bruh)
on 2009-03-03 11:24
Bruhtesfa Ebrahim wrote:
> Eric Blossom wrote:
>> On Thu, Feb 12, 2009 at 02:17:05PM -0800, Johnathan Corgan wrote:
>>> Actually, what you're now describing is more like not receiving any
>>> signal at all--the small line at DC is residual DC offset from the
>>> ADC, and the noise is simply that, noise that gets amplified according
>>> to the gain setting.  At a certain point the noise power exceeds the
>>> small DC offset.
>>>
>>> Johnathan
>>
>> Agreed about no (or low) signal, though I think the small line at DC
>> is a truncation artifact in the FPGA.
>>
>> Eric
>

Hey all,

What I understand from a repeated experiment is, only the carrier
frequencies which undergo direct downconversion by the
daughterboard(frequencies for which DDC=0 in FPGA) show such a
behaviour. For instance, for f=5.76GHz there is the problem stated
above, but for f=5.7605GHz there is no such a problem.

But, why such dependence for the direct down conversion frequencies?
I am using the frequencies like, f=5.7605GHz for now.But,will there be
any effect on my output due to using these fractional frequencies?


Bruhtesfa,
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